A truly vegan diet is now a possibility due to our increased nutritional knowledge and also our ability to obtain any kind of food at any time of year thanks to global trade. Isolated societies by necessity had to survive with whatever plant or animal foods were locally available. Only fairly recently have we had the ability to pursue a healthful pure vegetarian diet.

While the different societies in the book are not strictly vegan (and John truthfully reports this), they do consume a mainly plant-based diet, since that is the type of food at their disposable. It's a happy coincidence for them that these food choices have been a contributing factor to their longevity. John may seem to soft-sell the vegan diet, but I'm sure he wants this book to have universal appeal. Maybe he feels that some might be initially "scared off" due to the V-word. You don't have to read between the lines, however, to see that in the book a vegetarian or vegan diet is strongly emphasized for healthful longevity.